There are probably at least three times as many warnings in the New Testament about the way we speak of others as there are about sexual sin. Just an estimate.
I'm not 100% certain about this, but the pelvic issues appear to be more popular, or at least more popularly debated. Even a good few hours of Catholic Answers probably wouldn't turn up a real debate about how much malignant gossip is okay, presumably because we agree that the answer is "none."
The practice, as is often the case, doesn't match the theory. We say things that don't warrant saying. We damage reputations. Sometimes we're a bit more straightforward and we'll talk down to someone to their face. At least that's a little more honest. The things we say are, more often than not, wrong to say even when they happen to be true.
Those more generous among us may limit themselves to only cruel thoughts. This kindness limits the damage done to other hearts and souls, but certainly won't leave ours unscathed.
I believe the standard operating procedure for the inner life is to take every thought captive. May I humbly suggest that for certain thoughts, the Christian practice is better conceived of as catch-and-release?
I make no claim to a superior level of holiness in these matters. In fact, one of the reasons I am watchful (at least, I hope) on this subject is that I am painfully aware of my past failings in the area. I do better than I used to, but I remain, shall we say, short of virtue.
Let's talk witness for a moment. We can preach the Gospel to multitudes and fight all the culture wars we want, but if we don't have Love, all may be lost already. Truly there will be those who wouldn't know we are Christians by our Love even if they saw it right before their eyes, but the onus is on us to love them, and thus to leave them without excuse. This begins with the Love we have for each other.
Allow me another moment to clearly disown the following advice: "If you can't say anything nice, don't say anything at all." Unless our definition of practical is "having the property of stunting growth," this adage rightly fails the test of ethical practicality. It remains at best a childlike step to more mature and complete notion of charity, and at worst a hindrance to spiritual and societal growth.
We Christians, at least for the most part, should find people that we can be safe seeking counsel from, and trusting with less filtered thoughts (even, yes, about others' thoughts and words.) Let us choose those people wisely, though. We need people who will tell us when we're crossing the line.
Clearly there are times when conflict should be addressed face-to-face, even times when some rudeness or harshness is in order. The example of Truth Himself seems pretty clear there. Worth noting, though: Jesus isn't big on behind-the-back name-calling. If He's rude to someone, He'll oppose them to their face. (Paul took after Him.)
However, when we must risk cruelty to others, in person or from afar, in public or in private, let us be clear about the danger and take the risk with fear and trembling. Let us remember that words spoken about another--face to face, behind their backs, in speech or laid out as pixels, or even only in our minds--can never be erased from the memories of those who hear them. The danger is certainly one of wounded charity, but without flattering all crass gossip with the title of "mortal sin," we have to recognize that lesser sins will grow up someday.
That, at least, is worth thinking on before we speak.